1. Field of the Invention
This invention relates generally to vacuum systems and, more particularly, to cryopump vacuum systems used in conjunction with semiconductor manufacturing equipment.
2. Description of the Related Art
Cryopumps are in common use with semiconductor manufacturing equipment. For example, in a physical vapor deposition (PVD) system, a cryopump is used to pump-down a processing chamber to a pressure typically of about 10.sup.-8 torr. The cryopump must be able to accomplish this task without introducing substantial amounts of contaminants into the processing chamber.
In FIG. 1, a prior art cryopump 10 is coupled to a port 12 of a processing chamber 14 by a gate valve assembly 16. The processing chamber 14 may be, for example, a PVD processing chamber. Cryopumps are also used to pump-down chambers of other types of semiconductor manufacturing equipment. A cryopump 10 typically includes a substantially cylindrical casing 18 having an inlet 20 surrounded by a flange 22.
The cryopump 10 is provided with an inlet conduit 24 and an exhaust conduit 26. The inlet conduit 24 opens on the chamber 28 of the cryopump 10 and is typically provided with a shut-off valve 30. The exhaust conduit 26 also opens on the chamber 28, and is coupled to a mechanical pump 32 by a shut-off valve 34. The inlet conduit 24 allows the introduction of a purging gas (such as argon) into the chamber 28. The exhaust conduit 26 and pump 32 allow the removal of gases within the chamber 28.
Disposed within the chamber 28 of cryopump 10 are a number of chevrons 36a, 36b, 36c, and 36d. The chevrons are used to disperse gases flowing into inlet 20 within the chamber 28 and comprise a 80.degree. K condensing array or "80.degree. K array." The functioning of the 80.degree. K array will be discussed subsequently. Also disposed within the chamber 28 of cryopump 10 are a number of inverted cups generically referenced at 37. These inverted cups comprise a "15.degree. K array", which will also be discussed subsequently. The 15.degree. K array and the 80.degree. K array are surrounded by a cylindrical 80.degree. K radiation shield 39, and the 15.degree. K array is supported by a cold-head cylinder 41. The cold-head cylinder 41 is supplied with pressurized helium gas at an inlet 43a, and exhausts the helium gas at an outlet 43b. The cold-head cylinder 41, when supplied with the pressurized helium gas, cools the 15.degree. K array to about 15.degree. K, and cools the 80.degree. K which is supported above the cold-head cylinder 41 and the 15.degree. K array 37 to about 80.degree. K. That is, the 15.degree. K array is cooled to the neighborhood of the temperature of liquid helium, and the 80.degree. K array is cooled to the neighborhood of the temperature of liquid nitrogen.
As noted, cryopump assembly 10 typically includes both a 15.degree. K array and a 80.degree. K array. The 15.degree. K array typically takes the form of inverted cups provided with activated charcoal on their under-sides, and is super-cooled to about 15.degree. Kelvin by the cold head cylinder 41 such that the activated carbon "pumps" light gases, namely, helium, hydrogen and neon, through a chemical adsorption process. The 80.degree. K array typically takes the form of concentric metal chevrons, e.g. chevrons 36a-36d, and is operative to pump the heavier gases, such as nitrogen, oxygen, carbon monoxide, carbon dioxide, etc. by a chemical absorption process.
A new, or regenerated, cryopump is quite efficient, and can provide an ultrahigh purity vacuum of about 10.sup.-8 torr. The ultimate vacuum level attainable by the cryopump 10 is generally limited by its ability to pump hydrogen (H.sub.2). The 15.degree. K array of the cryopump 10 pumps hydrogen relatively slowly, which may allow hydrogen to integrate itself into a film being formed on a semiconductor wafer within processing chamber 14. This is due, in great extent, to the convoluted path that the hydrogen must make to the activated charcoal on the underside of the inverted cups 37 of the 15.degree. K array, resulting in very low "conductance" between these charcoal surfaces and the process chamber 14. This inability to effectively pump hydrogen is particularly problematical in PVD machines where the H.sub.2 can get "sputtered" into the film, thereby degrading the film quality.
Hydrogen is continually created within the processing chamber 14 due to, among other things, out-gassing from the stainless steel walls of the chamber 14 and through the decomposition of water on freshly deposited metal films such as aluminum. Since the 15.degree. K array is relatively inefficient in removing this hydrogen, it becomes quickly saturated, requiring "regeneration." Similarly, when the 80.degree. K array becomes saturated with heavier gases it, too, needs to be regenerated. This is typically accomplished by deactivating the cold head cylinder 41 and allowing the cryopump 10 to reach room temperature (approx. 25.degree. C.). At room temperature, the gases trapped within the 15.degree. K array and the 80.degree. K array are released within the chamber 28 and removed from the chamber by the pump 32. A purging gas, such as ultrahigh-purity (UHP) argon, may be released within the chamber 28 during this regeneration process to increase the pressure within the chamber 28, thereby increasing the heat transfer within the pump 32 and providing for a faster regeneration process.
A cryopump 10 is typically coupled to a flange 38 of processing chamber 14 by a gate valve assembly 16. The construction and use of gate valve assemblies is well known to those skilled in the art and, therefore, will not be discussed herein in detail. However, a typical gate valve assembly 16 includes a body 40 having an orifice 42 which can be aligned with the port 12 of the processing chamber 14 and with the inlet 20 of cryopump 10. The body 40 is typically provided with appropriate flanges and seals to provide a gas-tight connection between the cryopump 10 and the processing chamber 14. The gate valve assembly 16 includes a gate 44 and a gate-moving mechanism 46 which can move the gate 44 from the illustrated "open" position to a closed position as illustrated at 44'. When the gate 44 is in the closed position 44', a gas-tight seal is provided by a seal 48 to prevent gases and other materials from moving between the processing chamber 12 and the chamber 28 of the cryopump 10.
Because of the rapid saturation of the cryopump 10 with hydrogen and other gases, such as argon, from a PVD sputtering process, cryopumps have to be regenerated fairly frequently. For example, a cryopump coupled to a PVD machine will have to be regenerated from time to time. This is rather a costly procedure because the semiconductor manufacturing equipment must be taken "off line," thereby slowing or stopping the semiconductor manufacturing process.
It has been suggested that another type of pump known as the non-evaporable getter (NEG) pump be used in combination with cryopumps in an attempt to solve this problem. See, for example, "Non-evaporable Getter Pumps for Semiconductor Processing Equipment," by J. Briesacher et al, Journal of Ultraclean Technology, vol. 2, no. 1, 1990. However, as will be discussed subsequently, such combination pumps have been found in the prior to be impractical.
As well known to those skilled in the art, getter pumps utilize "gettering" materials comprising certain metal alloys which have a chemical affinity for particular gases. For example, a metal alloy including 70 percent Zr, 24.6 percent V, and 5.4 percent Fe, has a strong affinity for most gases other than noble gases. These "gettering" materials can therefore be used to quickly "pump" hydrogen through chemical adsorption.
While it is theoretically desirable to combine a cryopump with a getter pump, prior art solutions have been found to be less than desirable. For example, a getter pump could be provided in conjunction with a cryopump, such as by providing a getter pump next to cryopump 10 and mechanical pump 32 of FIG. 1. However, this leads to "form factor" problems because there is often not enough space around a piece of semiconductor manufacturing equipment to accommodate both a cryopump and a getter pump, along with their associated support hardware.
Another solution that has been suggested is to place the active elements of a getter pump within the chamber of a cryopump. However, such a solution tends to be impractical because of the incompatible operating and regenerating cycles of getter pumps and cryopumps. For example, the active elements of a getter pump operate best at about room temperatures, while the active elements of the cryopump operate at cryogenic temperatures, such as 15.degree. K and 80.degree. K. Furthermore, since the cryopump elements require frequent regeneration, the getter pump elements would have to be regenerated at the same frequency. This is a problem because getter pump elements can typically only be regenerated ten or so times, while cryopump elements can be regenerated hundreds of times. This would result in the rapid destruction of the expensive gettering material. Alternatively, if the gettering material were removed from the cryopump assembly prior to the regeneration of the active elements of the cryopump, the cryopump assembly would have to removed and replaced from the apparatus to which it is attached in a time-consuming and potentially system-contaminating procedure.
In U.S. Pat. No. 5,357,760 of Higham a combination cryopump/getter pump is disclosed including a pumping structure having an integral two-stage pump. The first-stage pump is a cryogenic pump having a pump chamber and cryo-arrays mounded on an expander for cryo-condensation of the principal gases present in the vacuum chamber. The second stage pump operates at room temperatures and includes one or more getter pumps whose principal function is to remove hydrogen molecules. A unitary housing is provided to enclose "in a single body" the first pumping stage and the second pumping stage. Therefore, the active elements of a getter pump are within the chamber of a cryopump, as described previously.
The Higham pump therefore has the aforementioned problem of having its cryogenic pump elements and the getter pump elements exposed to the same thermal and atmospheric environment. Since the cryogenic pump elements operate at cryogenic temperatures, and since the getter pump elements operate at near room temperature, the getter pump elements must be thermally shielded from the cryogenic pump elements, decreasing conductance. This conductance is also reduced due to the placement of the getter materials at the bottom of the pump. It should further be noted that the Higham pump eliminates the 15.degree. K array and, therefore, cannot pump neon or helium. The apparent reason to eliminate the 15.degree. K array is to eliminate the potential contamination of an integrated circuit manufacturing process by the charcoal in the array. Also, since the cryogenic pump elements are typically regenerated more frequently, it is necessary to regenerate the getter elements more frequently than would be otherwise required due to the sharing of the same pumping chamber, as described previously. In particular, the high temperatures used for getter regeneration (e.g.&gt;450.degree. C.) will irreversibly damage cryopump elements, especially the Indium gaskets that they typically use. In addition, the high temperatures could damage the refrigeration system of the cryopump.
The prior art, therefore, does not disclose a combination cryopump/getter pump which meets the required form factor for use with semiconductor manufacturing equipment, which is easily used and maintained, and which addresses the special operating and regeneration problems of both cryogenic pump elements and getter pump elements.